Ahavat Yonatan offers the following answer: “Abraham circumcised Ishmael when he became bar mitzvah, which was [actually] when he was 13 years old and a day. [Citing Tractate Niddah 47b, he explains that this is] because the [reckoning of] years for a son [reaching maturity at 13]…are counted me’et l’et, for days and not for hours.”
In other words we count “from time to time,” meaning from the beginning of the actual 24-hour period from the time of his birth. For example, if a child is born at 7:00 p.m. and tzeit ha’kochavim – the point at which it is surely evening – begins at 8:17 pm, we don’t reckon his attainment of maturity at 7:00 p.m., to the day, 13 years later. Rather, we finish off that day and enter the next day, which is when he officially becomes a bar mitzvah.
Tosafot (Rosh Hashanah 10a sv “ben”) explain that we do this in order to complete me’et l’et for his maturity. We don’t consider two years and one day as three years of age in regards to a maiden’s betrothal. Rather we require a full me’et l’et – three years plus a part of the next day.
Yet, in actuality, that point is now in the fourth year of the maiden in the case cited by Tosafot, or the 14th year in our case – Ishmael attaining maturity. Ahavat Yonatan explains that Abraham sought to circumcise Ishmael at the completion – i.e., the full measure – of his maturity so that he would be praiseworthy for not resisting being circumcised even though he was already beyond the authority of his father and thus could have resisted.
Rabbi Levine cites Terumat Hadeshen in his explanation of Rashi (supra 16:16), who notes the dispute regarding a minor child’s attainment of maturity – whether one reckons from et l’et for days or hours. According to the way we rule, one who was born on the eve of Rosh Hashanah, even at the end of the day, becomes bar mitzvah on Rosh Hashanah because he completed a full 13 years at the end of Rosh Hashanah evening. Thus, at the start of Rosh Hashanah he is already considered as a 14-year-old. This is according to the view (Rosh Hashanah 2b) that one day into the year is considered a year, because part of a year is like an entire year. This rule is inferred from the Torah’s second mention of “b’etzem hayom ha’zeh – on that very day.”
We mentioned previously that in his dialogue with Isaac, Ishmael argued that his act of submitting to the brit willingly is meritorious and proves his right to the spiritual inheritance of Abraham. It seems that Ishmael was very proud of performing a brit at his age. Accordingly, I submit that what Ishmael transmitted to his progeny is not a rule to perform circumcision in the manner that Isaac’s progeny do – at eight days – but rather proudly at an older age of awareness.
Some Arabs claim that Ishmael was sacrificed at the Akeidah, not Isaac. But the Torah predates the Koran by some 2,000 years; where were those who wish to rewrite Moses’ book, given before one-and-a-half million witnesses, all those centuries? Ishmael, who died a righteous man, never boasted that he was nearly offered as a sacrifice. Rather, his boast was that he was circumcised at age 13 and a day. That was what he conveyed to his progeny, who actually cling, somewhat, to that instruction.